21 replies to this topic

[asade abiodun]

In sizing multiple relief valves, should each valve be sized for full relief load? OR the Total relief load should be shared between the multiple valves?

Please, i need clarification.

[Zauberberg]

What the common sense is telling you? Is it logical to have 5 or 10 relief valves installed for depressurization of a single unit/circuit, and with each RV rated for full relief load?

[asade abiodun]

What the common sense is telling you? Is it logical to have 5 or 10 relief valves installed for depressurization of a single unit/circuit, and with each RV rated for full relief load?

With all due respect, it is the challenge at hand that makes me ask the question. The load to be relief is higher than a standard RV could take, and i am asking the question to seek opinion on dividing the relief load between 2 RVs. I never intend to go more than 2 RVs.

I hope that has clear your point.

[fallah]

The load to be relief is higher than a standard RV could take, and i am asking the question to seek opinion on dividing the relief load between 2 RVs.

You have to share the total load between two PSV's with stagerred set points.

Regards

[Qalander (Chem)]

fallah Has given the correct reply; but you have to be very clear about set pressures

(i.e. pop and re-seat pressures) for each of the installed relief valves and questions e.g.as follows(but not limited to these) may have be satisfied

• what will be the residual pressure during reliving?
• how much over pressure the vessel might be subjected to?
• what is the M.A.W.P.& what is the maximum design pressure at what temperature?
• what-if any (temperature related) pressure correction may have to be applied or calculated;while considering various credible scnario(s)!
• which specific or typical case is considered as governing case for design basis

Hope all this proves useful and guiding for you!

[asade abiodun]

Thanks All,
Your inputs are highly appreciated.

Fallah, sharing the load between the two RVs tells me that the system would handle the relief loads at an interval. Should the flare KO drum and flare system be designed to handle the relief load of one RV. This would minimize the KO drum size.

[fallah]

Fallah, sharing the load between the two RVs tells me that the system would handle the relief loads at an interval. Should the flare KO drum and flare system be designed to handle the relief load of one RV. This would minimize the KO drum size.

Flare KO drum and flare system should be designed to handle the relief load of two PSV's at rated flow and at rated flow both PSV's would be in sevice with their full capacities.
Actually,in this case rated flow shared between two PSV's because one PSV disables to handle total rated flow.

Edited by fallah, 22 October 2010 - 02:54 AM.

[asade abiodun]

Fallah,

Designing the flare KO drum for the rated flow gives a higher vessel dimension. The rated flow through the RVs are equivalent to the total rated flow (1,617,228 kg/h) through a high pressure compressor.
The challenge is how to minimize the Flare KO drum dimension.

What is your opinion on this?

Edited by Art Montemayor, 22 October 2010 - 07:49 AM.

[fallah]

Fallah,

Designing the flare KO drum for the rated flow gives a higher vessel dimension. The rated flow through the RVs are equivalent to the total rated flow (1,617,228 kg/h) through a high pressure compressor.
The challenge is how to minimize the Flare KO drum dimension.

What is your opinion on this?

Seems your system is just containing single relief device (a PSV splitted to two PSV's due to PSV capacity limitation).If so,you can use required flowrate for KO drum sizing with spring loaded PRV,provided that the initial flowrate wouldn't be higher than required flowrate.

[Lowflo]

Designing the flare KO drum for the rated flow gives a higher vessel dimension. The rated flow through the RVs are equivalent to the total rated flow (1,617,228 kg/h) through a high pressure compressor.
The challenge is how to minimize the Flare KO drum dimension.

What is your opinion on this?

Wow....that's a big load from a single source. This will clearly require multiple PSVs. Alternatively, this a textbook case for HIPS if the client has a good integrity management program. If they don't then stick with conventional relief devices.

I was just going through the interesting topic and have relevant query, I am sizing PSV on nitrogen vessel and has multiple PSV installed. I considered block outlet as credible scenario however I am bit confused whether to consider the fire as credible scenario since the stored gas is inert gas but the tank installed in white oil storage area and fire can take place surrounding area of the vessel.

[fallah]

I was just going through the interesting topic and have relevant query, I am sizing PSV on nitrogen vessel and has multiple PSV installed. I considered block outlet as credible scenario however I am bit confused whether to consider the fire as credible scenario since the stored gas is inert gas but the tank installed in white oil storage area and fire can take place surrounding area of the vessel.

kb_jadhav,

Fire should be considered as a credible scenario in your case, because of overpressure due to gas expansion.

Fallah

[Lowflo]

kb_jadhav,

First, recognize that a PSV is inadequate at protecting a gas-filled vessel when it's exposed to fire. Regardless of the PSV size, the vessel wall temperature will continue to rise until it eventually fails, and it won't take long for that to happen (see API-521). Consequently, I would size the PSV for the blocked-in condition, and use other protective measures to address this fire exposure risk. Those are: (1) water spray, (2) automatic de-pressurization, (3) fire resistant insulation.

Consider the size of the vessel and the pressure in it. In other words, assess the consequences of vessel failure. Based on that, determine if other protective measures are needed. If so, consider which of those three, or any combination of them, is appropriate based on that risk.

A rupture disk is another alternative, if the owner is willing to consider a disk on this vessel. But, if you're going to use a PSV, there's little value in worrying about how large to make it for this fire scenario. In that situation, the size of that PSV will make very little difference.

[fallah]

Consequently, I would size the PSV for the blocked-in condition, and use other protective measures to address this fire exposure risk. Those are: (1) water spray, (2) automatic de-pressurization, (3) fire resistant insulation.

Lowflo,

In a multiscenarios case (e.g. the case under discussion) relevant PSV should be sized based on governing scenario, even the govening scenario to be fire and protective measures would be available.

Fallah

[Lowflo]

Fallah,

If you meant to say the user "can" size the PSV for the fire case (vapor expansion), then that is true, but it's not true to say the user "should" do so.

There might be some cases in which sizing for vapor expansion due to fire exposure makes sense, but in the great majority it doesn't. It perpetuates the false belief that the vessel is adequately protected by a PSV alone, when in reality the PSV adds no real safety value while wasting time and money.

Regardless of whether the relief designer proceeds with sizing for vapor expansion, he should clearly document and inform the equipment owner that the PSV can't provide any meaningful protection from fire exposure. Therefore the owner needs to consider whether the risks justify alternate layers of protection.

[fallah]

Fallah, If you meant to say the user "can" size the PSV for the fire case (vapor expansion), then that is true, but it's not true to say the user "should" do so. There might be some cases in which sizing for vapor expansion due to fire exposure makes sense, but in the great majority it doesn't. It perpetuates the false belief that the vessel is adequately protected by a PSV alone, when in reality the PSV adds no real safety value while wasting time and money. Regardless of whether the relief designer proceeds with sizing for vapor expansion, he should clearly document and inform the equipment owner that the PSV can't provide any meaningful protection from fire exposure. Therefore the owner needs to consider whether the risks justify alternate layers of protection.

Lowflo,

Don't remember we were discussing on kb_jadhav on 11th post of this thread.
Anyway, i think user should do the sizing of PSV for fire case if it would be governing case among some applicable scenarios (or even when fire case is only applicable scenario) because of code requirement.

Just in cases that fire case is only applicable scenario, some companies have their own practices (affected by economical considerations) in a spectrum from not considering any PSV to considering a small size PSV to show not avoiding code requirement.

Fallah

Edited by fallah, 29 October 2011 - 03:42 AM.

[Lowflo]

Fallah, If you meant to say the user "can" size the PSV for the fire case (vapor expansion), then that is true, but it's not true to say the user "should" do so. There might be some cases in which sizing for vapor expansion due to fire exposure makes sense, but in the great majority it doesn't. It perpetuates the false belief that the vessel is adequately protected by a PSV alone, when in reality the PSV adds no real safety value while wasting time and money. Regardless of whether the relief designer proceeds with sizing for vapor expansion, he should clearly document and inform the equipment owner that the PSV can't provide any meaningful protection from fire exposure. Therefore the owner needs to consider whether the risks justify alternate layers of protection.

Lowflo, Don't remember we were discussing on kb_jadhav on 11th post of this thread. Anyway, i think user should do the sizing of PSV for fire case if it would be governing case among some applicable scenarios (or even when fire case is only applicable scenario) because of code requirement. Just in cases that fire case is only applicable scenario, some companies have their own practices (affected by economical considerations) in a spectrum from not considering any PSV to considering a small size PSV to show not avoiding code requirement. Fallah

Fallah,

For all of the codes that I'm familiar with, and I'm familiar with most, the valve sizing decision is intentionally left to the user. This is true for ASME, API, and the harmonized European Union standards. If the user thinks, as I do, that sizing for fire exposure to vapor filled vessels makes no sense, then that decision is in full compliance with codes and guidance standards.

There's only one exception, which applies exclusively to low pressure tanks (not pressure vessels) located in the US. OSHA 1910.106 mandates that all storage tanks have a fire-sized ERV, unless they contain Class IIIB materials and are not exposed to a spill from a tank containing Class IIIA or lower. There are no such mandates for pressure vessels, nor does EN 14015 (EU low-pressure tank standard) have such a requirement.

Lowflow,

I agree to the above reply given by you, however I need to check the API-521, why we are considering fire case for PSV sizing if it doesn't protect the vessel, and depressurization is only option during fire scenario. PSV will pop only when the vessel attains set pressure, since it will not relieve all the pressure inside the vessel even in case of fire. Providing other options like (1) water spray, (2) automatic de-pressurization, (3) fire resistant insulation, seems better options.

We can think of providing alternative options like rupture disk however in case of block outlet case and other possible scenario but there is loss of inventory if any of the contingencies happened. The possibility of these contingencies is more than fire and hence often there will be loss of inventory which is not accepted. What we can think of providing PSV for other scenario and rupture disk for fire scenario set at higher set pressure than PSV set pressure but less than or equal to design pressure of the vessel (set pressure of rupture disk is more than relieving pressure of PSV) so that inventory inside the vessel during other contingencies can be saved and it will be let off during fire but here we can protect the vessel or avoid other damages OR explosion.

Even it is economic as PSV size will be smaller than PSV sized for fire case.

Presently I have sized PSV for fire case but would like to take concurrence of Client for providing rupture disk.

kb_jadhav

[fallah]

Fallah,

For all of the codes that I'm familiar with, and I'm familiar with most, the valve sizing decision is intentionally left to the user. This is true for ASME, API, and the harmonized European Union standards. If the user thinks, as I do, that sizing for fire exposure to vapor filled vessels makes no sense, then that decision is in full compliance with codes and guidance standards.

Lowflo,

As a sample, please note to the attached as a page from ASME VIII in which sizing of PSV in fire case based on maximum 21% overpressure is mandated.

Fallah

Attached Files

•  ASME Sec VIII-Pressure Relief Requirement in Fire Case .pdf   53KB   89 downloads

[Lowflo]

Fallah - You have misunderstood that paragraph. I can assure you that ASME has no intention of interferring with the user's perogative to determine the sizing scenario.

[fallah]

Fallah - You have misunderstood that paragraph. I can assure you that ASME has no intention of interferring with the user's perogative to determine the sizing scenario.

Lowflo,

I didn't claim that ASME has intended of interferring with the user's prerogative in sizing scenario determination. Let me clarify the matter in more detail:

Sizing scenario has been determined based on highest calculated orifice area among those which calculated for all applicable scenarios. If fire case would be among applicable scenarios there would be three different situations:

1- Fire case is only applicable scenario- In this situation PSV should be sized based on maximum 21% overpressure as ASME mentioned.
2- Fire case is among applicable scenarios but isn't governing case- In this situation selected/sized PSV would certainly cover fire case relief, hence ASME requirement regarding fire case to be met.
3- Fire case is among applicable scenarios and also is governing case- In this situation the required PSV should be sized based on fire case. Obviously, the sized PSV would cover all relief values of other remained applicable scenarios.

As you see in all situations involving fire case ASME requirement have been met and in first and third one the user have to select the fire case as sizing scenario for relevant PSV without ASME's direct interferring on the matter.

Fallah

Edited by fallah, 31 October 2011 - 02:55 AM.

[Lowflo]

Fallah.....we're going around in a circle.

Yes, the user can choose to size for fire in this case. Alternatively, the user can conclude that that makes no sense and size for some other scenario, or install an arbitrarily sized PSV if there are no other scenarios. All of the above are in full compliance with ASME and other codes.